The proposed research is directed toward investigating the molecular mechanism of signal transduction induced by the herpesvirus saimiri (HVS) tyrosine kinase interacting protein (Tip). Our work to date has defined that HVS Tip is involved in the establishment and maintenance of persistent infection in the natural host, the squirrel monkey, and required for primary lymphoid cell immortalization in culture and lymphoma induction in the non-natural host, the common marmoset. Our biochemical analysis also demonstrates that Tip targets Lck tyrosine kinase, STAT transcription factor, p80 endosomal adaptor, and Vps35 retromer subunit to down regulate T cell receptor (TCR) signal transduction and to elicit STAT-mediated survival signal. Based on our preliminary results, we hypothesize that the signaling and targeting functions of HVS Tip rely on sequential functionally and genetically separable mechanisms that comprehensively inhibit TCR signal transduction: Tip interaction with Lck inhibits and recruits the TCR complex to lipid rafts, Tip interaction with p80 subsequently induces the aggregation and internalization of lipid rafts, and Tip interaction with retromer finally directs TCR complex to the lysosomes for degradation. On the other hand, the phosphorylation of Tip by Lck recruits and activates STAT/Nmi transcription factor complex to provide survival signal. Consequently, Tip expression not only deregulates TCR signal transduction and expression but also activates STAT-mediated survival signaling to establish and maintain viral persistent infection in squirrel monkeys and to contribute to the development of lymphoproliferative disease in common marmosets. Our biochemical and cell biological studies will define in greater detail the mechanisms used by Tip to deregulate TCR signal transduction and to elicit STAT-mediated lymphocyte activation. To correlate the effects of Tip on cellular signal transduction with viral pathogenesis and persistent infection, we will test whether recombinant HVS containing wt Tip or its mutants is able to induce the primary lymphoid cell immortalization and lymphoma in common marmosets and to establish the persistent infection in squirrel monkeys. With well-established in vitro and in vivo experimental conditions, the proposed study will detail the roles of Tip in the deregulation of cellular signal transduction and the induction of survival signal, which consequently contributes to viral persistence and pathogenesis.